A guide to hiring SQL developers
SQL is the language most businesses use to ask questions of their data. Orders, customers, invoices and web traffic often sit in tables, and someone has to write the statements that turn those tables into answers. This guide explains what a SQL developer does, how the role differs from nearby roles and how to set up the work so it goes smoothly. It suits a founder whose reports are slow as well as a company that wants a steady person on its data.
What SQL developers work on
Most SQL work falls into four groups. Queries pull the rows you need and combine them across tables. Reports and views package those queries so dashboards and spreadsheets always show the same figures. Design work covers the tables, keys and rules that keep data tidy. Maintenance covers cleaning, moving and fixing data when something goes wrong. A small project may need one person for all four, while a larger one splits them.
How this role differs from nearby roles
A SQL developer works on the questions and the structure of the data. Our data engineer page is about the wider platform: pipelines, scheduled jobs and warehouses. Our database engineer pages are about running databases safely, with backups, failover and migrations. If your data lives in one engine and you want deeper help with it, our PostgreSQL developer page covers that engine in detail. A back-end team may also need back-end developers who write the application code around the database.
Common SQL engines
- PostgreSQL is an open source engine known for strict standards support and rich data types.
- MySQL is a widely used open source engine behind many websites and online shops.
- SQL Server is Microsoft’s database, common in companies that use other Microsoft tools.
- Oracle Database is common in large organizations with long-running systems.
- Cloud services such as Amazon RDS, Google Cloud SQL and Azure SQL run these engines for you.
Say early which engine and version you use. It changes the functions, the tuning tools and the way code is written.
What to decide before you hire
- Which questions the business needs answered, and who will read the answers.
- Which engine and version you run, and where it is hosted.
- Whether the work is a one-off clean-up or an ongoing role.
- How much access the developer needs, and whether a copy of the data will do.
- Where the finished queries and notes will live, such as a shared code repository.
Reading a SQL developer’s profile
Ask for samples. A good candidate can share a query they wrote, with names changed, and explain each part. Look at how it is laid out, whether the names make sense and whether there are notes about the edge cases. Be careful with profiles that list every database but cannot say which problem they solved on each. Ask about a time a number turned out to be wrong and how they found out. A person who can explain a mistake plainly is easier to trust than one who has none to share.
Interview questions to try
- What is the difference between an inner join and a left join, and when does the choice change a total?
- How would you find the second highest order for each customer?
- A report that used to finish quickly now takes minutes. What do you check first?
- When would you add an index, and what does it cost you?
- How do you make sure a new query returns the same figures as the old one?
Listen for clear steps and examples. Doubt anyone who says a change cannot affect other reports.
Run a small paid task
Give each finalist a short brief that matches your data. Good examples are a monthly sales report from three tables, a clean-up of duplicate customer records, or a slow query to speed up with an explanation. Pay for the time. Compare how readable the query is, how the person checks the result and how they handle questions about what a column means.
Match experience to the stage
At the early stage you need someone who can turn questions into queries quickly and keep the design simple. As the data grows you need a developer who is strong at tuning, writes careful notes and works well with analysts. In a mature company you need steady care for shared reports, clear definitions and a plan for changes that touch many teams. Job titles differ between companies, so ask what each person owned rather than relying on the title.
Give them tools and a rhythm
Give your developer a limited account, a place to run queries safely and a repository for the finished code. Production passwords should stay with your company. In the first month, talk at least once a week and read each query together. After that, a weekly update and a second pair of eyes on any change to a shared report are enough.
Working with analysts and your own team
A SQL developer works well when the people who use the data can say what they need in plain words. Hold a short session where analysts, finance and operations agree on what common terms mean, such as an active customer or a completed order. Write those definitions down next to the queries. When two reports disagree, the notes show where the difference comes from.
Mistakes to avoid
- Letting several versions of the same report grow, each with slightly different rules.
- Tuning a query before checking that it returns the right rows.
- Giving write access to production data when read access would do.
- Keeping important queries only in someone’s personal tool.
- Letting a contractor own your database accounts or your only copy of the code.
Need a wider data team or one specific engine? Read about how to hire data engineers or PostgreSQL developers, and see how hiring works on our pages.